This invention relates to the field of surface treatment of shaped metal components of aluminum or aluminum alloys and of tin-plated steel (tinplate). It relates in particular to beverage and food cans of these materials. The invention pursues the aim of providing the can surfaces, during the can production process, with a corrosion-protective layer which can be enameled over, which facilitates running off of water and in particular leads to a reduction in the coefficient of friction between cans in contact with one another and therefore facilitates transport of the cans on conveyor belts, the porosity of a later coat of enamel being reduced at the same time.
Cans of tin-plated steel (tinplate) and of aluminum (or of aluminum alloys, which are included under "aluminum" in the following for simplicity) are widely used for storage of foods and especially of beverages. In the can production process, after shaping these are usually washed; for this, acid or alkaline cleaning agents, for example, are commercially obtainable. These cleaning agent solutions must have an adequate dissolving power for the metals in question in order to remove abraded metal from the cans effectively. The can surface can itself be roughened by the attack on the metal; this increases the friction between cans in contact. The speed of transport of cans on conveyor belts is thereby reduced, and especially at points where a back-up of the cans develops due to separation of the cans, can transport may become blocked completely. Since the capacity of the production line is thereby reduced, efforts are made to condition the can surfaces such that friction between cans in contact is as low as possible.
However, application of a friction-reducing layer should not mean that the adhesion of enamel coatings, labelling or other coatings applied for corrosion protection and/or for decorative reasons suffers. Furthermore, the coating must ensure that requirements in respect of corrosion resistance, which vary according to the contents of the cans, are met. Only those active compounds which are generally ecologically acceptable and in particular acceptable for foods should be employed here. For example, efforts are made to avoid chromium-containing reagents for environmental reasons.
Various chromium-free processes for the surface treatment of aluminum, which as a rule employ inorganic acids, in particular phosphoric acid, hydrofluoric acid or other sources of fluoride and/or complex fluorides and which operate with or without the additional use of organic polymers are known in the prior art. For example, U.S Pat. No. 4,992,116 describes an aqueous acid treatment solution which comprises phosphate, a fluoroacid of Zr, Ti, Hf or Si and a polyphenol compound which is a Mannich adduct of a substituted amine onto a polyalkylenephenol or a tannin. EP-B-8942 discloses treatment solutions, preferably for aluminum cans, comprising a) 0.5 to 10 g/l of polyacrylic acid or of an ester thereof and b) 0.2 to 8 g/l of at least one of the compounds hexafluorozirconic acid, hexafluorotitanic acid or hexafluorosilicic acid.
U.S. Pat. No. 4,470,853 discloses conversion solutions for aluminum which comprise, inter alia, 10 to 150 ppm of zirconium, 20 to 250 ppm of fluoride, 15 to 100 ppm of phosphate and 30 to 125 ppm of tannin. Their pH is in the range from 2.3 to 2.95. The use of tannin in the surface treatment of aluminum is also disclosed in DE-A-24 46 492, according to which aluminum is treated with an acidic, phosphate-containing solution which comprises a metal salt of tannin in amounts of between 0.1 and 10 g/l.
Various solutions have already been proposed for reducing the friction between aluminum cans during transport of the cans. For example, W091/14014 describes an aqueous solution which comprises ions of Fe, Zr, Sn, Al or Ce, metal-etching acids, such as, for example, hydrofluoric acid, alkoxylated phosphoric acid esters and a combination of alkoxylated alcohols and alkoxylated alkylphenols. WO94/01 517 describes a process for friction-reducing conversion treatment of metal cans in which alkoxylated or non-alkoxylated castor oil triglycerides, hydrogenated castor oil derivatives, alkoxylated or non-alkoxylated amine salts of fatty acids, alkoxylated or non-alkoxylated amino-fatty acids, alkoxylated or non-alkoxylated fatty amine N-oxides, alkoxylated or non-alkoxylated quaternary ammonium salts or water-soluble organic polymers are employed, in addition to inorganic metal compounds. Those amine oxides or quaternary ammonium salts in which at least one radical contains up to 20 carbon atoms are employed here. Amine compounds of this type are also employed in the context of the present invention. EP-A612 833 proposes a surface treatment with an ester between a polyglycerol and fatty acids for reducing friction.